SHEAR STRENGTH OF HIGH STRENGTH REINFORCED CONCRETE BEAMS

Abstract

Engineers have built structures with concrete strengths up to 11,000 psi by assuming the design rules developed for low strength concrete are also applicable to these new, higher strengths. This project investigated the suitability of the current American Concrete Institute shear design provisions for beams with concrete strengths in excess of 10,000 psi. Thirty-nine reinforced concrete beams with and without shear reinforcement were tested to determine their diagonal cracking and ultimate shear capacities. In beams without stirrups the shear span/depth ratio was held constant at 3.6, 2.5, or 1.5 while the concrete compressive strength was varried from 3000 psi to about 15000 psi. Concrete strength was also varied in beams with stirrups and with shear span/depth ratios of 3.6. Two additional series of beams were cast with preformed, smooth inclined cracks to remove aggregate interlock capacity. Design equations based on the test data are presented. Based on the results of this study, it was concluded that the ACI equations for predicting shear capacity of slender beams with or without stirrups and of deep beams without stirrups are conservative throughout the entire range of the concrete strengths tested. However, the actual factor of safety against shear failure is greatly affected by concrete strength and by shear span/depth ratio. The shear contributions due to aggregate interlock and dowel capacity were significantly affected by the concrete strength. (Author)